The invention provides a polystyrene ethyl isovalerate, and a synthetic method and an application thereof. The polystyrene ethyl isovalerate is prepared through a suspension polymerization reaction of ethyl isovalerate and styrene as reaction monomers with water as a dispersion medium in the presence of a dispersant polyvinyl alcohol and an initiator benzoyl peroxide. A test result shows that the polystyrene ethyl isovalerate prepared through the synthetic method can be completely dissolved in crude oil, the concentration of polystyrene ethyl isovalerate can be accurately determined through a gas chromatograph-mass spectrometer, the use amount is small, and horizontal well oil production profile monitoring needs are met.